Dehydration-mediated activation of the xanthophyll cycle in darkness: is it related to desiccation tolerance?

The development of desiccation tolerance by vegetative tissues was an important step in the plants’ conquest of land. To counteract the oxidative stress generated under these conditions the xanthophyll cycle plays a key role. Recent reports have shown that desiccation itself induces de-epoxidation of xanthophyll cycle pigments, even in darkness. The aim of the present work was to study whether this trait is a common response of all desiccation-tolerant plants. The xanthophyll cycle activity and the maximal photochemical efficiency of PS II (F _v/F _m) as well as β-carotene and α-tocopherol contents were compared during slow and rapid desiccation and subsequent rehydration in six species pairs (with one desiccation-sensitive and one desiccation-tolerant species each) belonging to different taxa. Xanthophyll cycle pigments were de-epoxidised in darkness concomitantly with a decrease in F _v/F _m during slow dehydration in all the desiccation-tolerant species and in most of the desiccation-sensitive ones. De-epoxidation was reverted in darkness by re-watering in parallel with the recovery of the initial F _v/F _m. The stability of the β-carotene pool confirmed that its hydroxylation did not contribute to zeaxanthin formation. The α-tocopherol content of most of the species did not change during dehydration. Because it is a common mechanism present in all the desiccation-tolerant taxa and in some desiccation-sensitive species, and considering its role in antioxidant processes and in excess energy dissipation, the induction of the de-epoxidation of xanthophyll cycle pigments upon dehydration in the dark could be understood as a desiccation tolerance-related response maintained from the ancestral clades in the initial steps of land occupation by plants.     

Desiccation tolerance in the vegetative tissues of the fern Mohria caffrorum is seasonally regulated

As there is limited information on the mechanisms of vegetative desiccation tolerance in pteridophytes, we undertook a comprehensive anatomical, ultrastructural, physiological and biochemical study on the fern Mohria caffrorum . Our data show that this species is desiccation-tolerant during the dry season, and desiccation-sensitive in the rainy season. This system allows the verification of protection mechanisms by comparison of tolerant and sensitive tissues of the same species at the same developmental age. Tolerant fronds acquire protection mechanisms during drying that are mostly similar to those reported for angiosperms. These include: (i) chlorophyll masking by abaxial scales and frond curling; (ii) increased antioxidant capacity that is maintained in dry tissues; (iii) mechanical stabilization of vacuoles in the dry state; (iv) de novo production of heat stable proteins (at least one identified as a putative chaperonin); (v) accummulation of protective carbohydrates (sucrose, raffinose family oligosaccharides and cyclitols). This study has implications for the biotechnological production of drought-tolerant crops, and allows speculation on the evolution of vegetative desiccation tolerance.     

Resurrection Plants and the Secrets of Eternal Leaf

Most higher plants possess a phase in their life cycle in whichtissues can survive desiccation. However, this is restrictedto specialized tissues such as seeds and pollen. Resurrectionplants are remarkable in that they can tolerate almost completewater loss in their vegetative tissues. The desiccated plantcan remain alive in the dried state for several years. However,upon watering the plants rehydrate and are fully functionalwithin 48 h. Underpinning this amazing ability is the capacityto accumulate large amounts of sucrose in the tissues. Thissugar has the property of stabilizing enzymes and cellular structuresin the absence of water. The sources of carbon that fuel sucrosesynthesis are not known, but temporary carbohydrate stores andphotosynthesis are the most likely candidates. On rewatering,the sucrose is metabolized rapidly as the tissues rehydrate.Increased expression of a number of genes in response to droughtstress have been noted. A number of these are associated withmetabolic pathways linked with primary carbohydrate metabolism.However, some genes related to LEA (Late Embryogenic Abundant)proteins have been isolated which suggests they too may playa role in maintaining tissue integrity during desiccation. Howthese mechanisms are integrated to enable resurrection plantsto survive desiccation is discussed. Copyright 2000 Annals ofBotany Company ABA, Craterostigma, desiccation tolerance, poikilohydric, resurrection.     

Freeze-substitution of dehydrated plant tissues: Artefacts of aqueous fixation revisited

Summary This investigation assessed the extent of rehydration of dehydrated plant tissues during aqueous fixation in comparison with the fine structure revealed by freeze-substitution. Radicles from desiccation-tolerant pea (Pisum sativum L.), desiccation-sensitive jackfruit seeds (Artocarpus heterophyllus Lamk.), and leaves of the resurrection plantEragrostis nindensis Ficalho & Hiern. were selected for their developmentally diverse characteristics. Following freeze-substitution, electron microscopy of dehydrated cells revealed variable wall infolding. Plasmalemmas had a trilaminar appearance and were continuous and closely appressed to cell walls, while the cytoplasm was compacted but ordered. Following aqueous fixation, separation of the plasmalemma and the cell wall, membrane vesiculation and distortion of cellular substructure were evident in all material studied. The sectional area enclosed by the cell wall in cortical cells of dehydrated pea and jackfruit radicles and mesophyll ofE. nindensis increased after aqueous fixation by 55, 20, and 30%, respectively. Separation of the plasmalemma and the cell wall was attributed to the characteristics of aqueous fixatives, which limited the expansion of the plasmalemma and cellular contents but not that of the cell wall. It is proposed that severed plasmodesmatal connections, plasmalemma discontinuities, and membrane vesiculation that frequently accompany separation of walls and protoplasm are artefacts of aqueous fixation and should not be interpreted as evidence of desiccation damage or membrane recycling. Evidence suggests that, unlike aqueous fixation, freeze-substitution facilitates reliable preservation of tissues in the dehydrated state and is therefore essential for ultrastructural studies of desiccation.Abbreviations LM light microscopy - TEM transmission electron microscopy - CF conventional (aqueous) fixation - FS freeze-substitution - ER endoplasmic reticulum     

Effects of Desiccation on Phosphorus and Potassium Acquisition by a Desiccation-tolerant Moss and Lichen

The hypothesis that desiccation-tolerant mosses and lichensmay be more responsive to nutrient inputs accompanying intermittentdesiccation than mesophytic forest species was investigatedemploying species from semi-arid grassland in Hungary. Shootapices of the moss Syntrichia ruralis and marginal lobes ofthe lichen Cladonia convoluta were maintained for 7 weeks undercontrolled conditions. They were cultivated with or withouta weekly application of the major inorganic macronutrients,and either under constant hydration or with one or two 24 hperiods of desiccation each week. Growth of S. ruralis was stimulatedby nutrient additions, but lower weight increments were achievedwith increasing frequency of desiccation. All samples of thelichen showed positive growth, and no significant treatmenteffects were detected. A large net uptake of P occurred in nutrient-treatedmaterial of both species that was unaffected by the impositionof desiccation treatments. A smaller net uptake of K into theintracellular fraction was also observed when nutrients wereapplied, but in the moss this was against a baseline of decreasingK content. In contrast, more of the original K content was retainedin C. convoluta. In neither species was any clear evidence foundfor inhibition of nutrient uptake by the desiccation episodes.It is suggested that the lack of growth response in the lichenarises from an inability to bring together the additional nutrients,presumably mainly absorbed by the mycobiont, with photosynthateproduced by the photobiont. Copyright 2000 Annals of BotanyCompany Cladonia convoluta, Syntrichia ruralis, desiccation tolerance, mineral nutrition, phosphorus, potassium     

Desiccation stress in recalcitrant Quercus robur L. seeds results in lipid peroxidation and increased synthesis of jasmonates and abscisic acid

In a series of experiments the desiccation-sensitive seeds ofQuercus robur were exposed to drying conditions both beforeand after a period of moist storage. Viability loss occurredat higher moisture contents in stored seed than in newly harvestedseeds. Measurements were made at intervals during desiccation.In both stored and unstored seeds viability loss was precededby an increase in the rate of ethane evolution, a commonly usedindicator of lipid peroxidation, and by an increase in electrolyteleakage indicative of membrane damage. Jasmonic acid (JA), itsmethyl ester (MeJA) and ABA were quantified in the same extractsfrom both cotyledonary and axis tissues. The concentration ofall three hormones was higher in the axis than in the cotyledonsof untreated seeds and were within the range of concentrationsquantified elsewhere in seed tissues from other species. Theconcentration of JA, MeJA and ABA progressively increased duringdrying in both cotyledons and axes of whole seeds and in excisedaxes prior to viability loss and then subsequently declined.The concentration of these hormones increased earlier duringdrying in stored seeds in line with their enhanced desiccationsensitivity. Exogenous JA, MeJA and ABA were shown to inhibit germination.However, none of these substances promoted ethylene evolution,which also inhibits germination of Q. robur seeds, or inducedsenescence-like deterioration. The results presented are discussed in relation to the natureof desiccation sensitivity and viability loss in Q. robur seeds. Key words: Quercus robur, seed, desiccation, jasmonates, abscisic acid     

Comparison of sucrose metabolism during the rehydration of desiccation-tolerant and desiccation-sensitve leaf material of Sporobolus stapfianus

Sucrose accumulated during dehydration is a major potential energy source for metabolic activity during rehydration. The objective of the present study was to investigate aspects of leaf sucrose metabolism during the rehydration of desiccation-tolerant Sporobolus stapfianus Gandoger (Poaceae) over a 10-day period. Comparison was then made to sucrose metabolism during the rehydration of both desiccation-tolerant excised leaf material (dehydrated attached to the parent plant) and desiccation-sensitive leaf material (dehydrated detached from the parent plant to prevent the induction of tolerance) over a 48-h period. The pattern of sugar mobilization and glycolytic enzyme activity during the rehydration of the desiccation-tolerant excised leaves was similar to that in leaves attached to the parent plants. Significant breakdown of sucrose was not apparent in the initial phase of rehydration, suggesting the utilization of alternate substrates for respiratory activity. The desiccation-tolerant excised tissues provided a suitable control to compare the metabolism of rehydrating desiccation-sensitive material. In contrast to the tolerant tissues, sucrose breakdown in the sensitive leaves commenced immediately after watering and the accumulation in hexose sugars was inversely proportionate to the decrease in sucrose content. Hexokinase (EC 2.7.1.1), PFK (ATP phosphofructokinase, EC 2.1.7.11), aldolase (EC 4.1.2.13), enolase (EC 4.2.1.11), and PK (pyruvate kinase, EC 2.7.1.40) activity levels were significantly lower in the desiccation-sensitive material during rehydration.     

Ecological correlates of seed desiccation tolerance in tropical African dryland trees

In the tropics, species with recalcitrant or desiccation-sensitive, Type III seeds are largely restricted to regions with comparatively high rainfall, because desiccation-induced seed death will be minimal in these environments. However, species with recalcitrant seeds do occur in drylands, although little is known about ecological adaptations to minimize seed death in these environments. Here we present data for the seed desiccation tolerance of 10 African dryland species and examine the relationships between seed size, rainfall at the time of seed shed, and desiccation tolerance for these and a further 70 species from the scientific literature. The combined data set encompasses species from 33 families. Three species (Syzygium cumini, Trichilia emetica, and Vitellaria paradoxa) had desiccation-sensitive seeds, and the remaining seven species investigated were desiccation-tolerant. The desiccation-sensitive species had large (>0.5 g) seeds, germinated rapidly, and had comparatively small investments in seed physical defenses. Furthermore, seed was shed in months of high rainfall (>60 mm). In comparison, for species with desiccation-tolerant seeds, seed mass varied across five orders of magnitude, and seed was shed in wet and dry months. Although infrequent in dryland environments (approximately 11% of the species examined here), species with desiccation-sensitive seeds do occur; large size, rapid germination, and the timing of dispersal all reduce the likelihood of seed drying. Furthermore, desiccation-sensitivity may be advantageous for large-seeded species by increasing the efficiency of resource use in seed provisioning.     

Desiccation Tolerance Studied in the Resurrection Plant Craterostigma plantagineum

This review will focus on the acquisition of desiccation tolerancein the resurrection plant Craterostigma plantagineum. Molecularaspects of desiccation tolerance in this plant will be comparedwith the response of non-tolerant plants to dehydration. Uniquefeatures of C. plantagineum are described like the CDT-1 (Craterostigmadesiccation tolerance gene-1) gene and the carbohydrate metabolism.Abundant proteins which are associated with the desiccationtolerance phenomenon are the late embryogenesis abundant (=LEA)proteins. These proteins are very hydrophilic and occur in severalother species which have acquired desiccation tolerance.     

Zygotic embryo cell wall responses to drying in three gymnosperm species differing in seed desiccation sensitivity

Plant cell walls (CWs) are dynamic in that they can change conformation during ontogeny and in response to various stresses. Though seeds are the main propagatory units of higher plants, little is known of the conformational responses of zygotic embryo CWs to drying. This study employed cryo-scanning electron microscopy to compare the effects of desiccation on zygotic embryo CW morphology across three gymnosperm species that were shown here to differ in seed desiccation sensitivity: Podocarpus henkelii (highly desiccation-sensitive), Podocarpus falcatus (moderately desiccation-sensitive), and Pinus elliottii (desiccation-tolerant). Fresh/imbibed (i.e. fresh Podocarpus at shedding and imbibed Pi. elliottii) embryos showed polyhedral cells with regular walls, typical of turgid cells with an intact plasmalemma. Upon desiccation to c. 0.05 g g^?1 (dry mass basis), CWs assumed an undulating conformation, the severity of which appeared to depend on the amount and type of dry matter accumulated. After desiccation, intercellular spaces between cortical cells in all species were comparably enlarged relative to those of fresh/imbibed embryos. After rehydration, meristematic and cotyledonary CWs of P. henkelii and meristematic CWs of P. falcatus remained slightly undulated, suggestive of plasmalemma and/or CW damage, while those of Pi. elliottii returned to their original conformation. Cell areas in dried-rehydrated P. henkelii root meristem and cotyledon were also significantly lower than those from fresh embryos, suggesting incomplete recovery, even though embryo water contents were comparable between the two states. Electrolyte leakage measurements suggest that the two desiccation-sensitive species incurred significant plasmalemma damage relative to the tolerant species upon desiccation, in agreement with the CW abnormalities observed in these species after rehydration. Immunocytochemistry studies revealed that of the four CW epitopes common to embryos of all three species, an increase in arabinan (LM6) upon desiccation and rehydration in desiccation-tolerant Pi. elliottii was the only difference, although this was not statistically significant. Seed desiccation sensitivity in species like P. henkelii and P. falcatus may therefore be partly based on the inability of the plasmalemma and consequently CWs of dried embryos to regain their original conformation following rehydration.     

The evolution of vegetative desiccation tolerance in land plants

Vegetative desiccation tolerance is a widespread but uncommon occurrence in the plant kingdom generally. The majority of vegetative desiccation-tolerant plants are found in the less complex clades that constitute the algae, lichens and bryophytes. However, within the larger and more complex groups of vascular land plants there are some 60 to 70 species of ferns and fern allies, and approximately 60 species of angiosperms that exhibit some degree of vegetative desiccation tolerance. In this report we analyze the evidence for the differing mechanisms of desiccation tolerance in different plants, including differences in cellular protection and cellular repair, and couple this evidence with a phylogenetic framework to generate a working hypothesis as to the evolution of desiccation tolerance in land plants. We hypothesize that the initial evolution of vegetative desiccation tolerance was a crucial step in the colonization of the land by primitive plants from an origin in fresh water. The primitive mechanism of tolerance probably involved constitutive cellular protection coupled with active cellular repair, similar to that described for modern-day desiccation-tolerant bryophytes. As plant species evolved, vegetative desiccation tolerance was lost as increased growth rates, structural and morphological complexity, and mechanisms that conserve water within the plant and maintain efficient carbon fixation were selected for. Genes that had evolved for cellular protection and repair were, in all likelihood, recruited for different but related processes such as response to water stress and the desiccation tolerance of reproductive propagules. We thus hypothesize that the mechanism of desiccation tolerance exhibited in seeds, a developmentally induced cellular protection system, evolved from the primitive form of vegetative desiccation tolerance. Once established in seeds, this system became available for induction in vegetative tissues by environmental cues related to drying. The more recent, modified vegetative desiccation tolerance mechanism in angiosperms evolved from that programmed into seed development as species spread into very arid environments. Most recently, certain desiccation-tolerant monocots evolved the strategy of poikilochlorophylly to survive and compete in marginal habitats with variability in water availability.     

The limits and frontiers of desiccation-tolerant life

Drying to equilibrium with the air is lethal to most speciesof animals and plants, making drought (i.e., low external waterpotential) a central problem for terrestrial life and a majorcause of agronomic failure and human famine. Surprisingly, awide taxonomic variety of animals, microbes, and plants do toleratecomplete desiccation, defined as water content below 0.1 g H₂Og^–1 dry mass. Species in five phyla of animals and fourdivisions of plants contain desiccation-tolerant adults, juveniles,seeds, or spores. There seem to be few inherent limits on desiccationtolerance, since tolerant organisms can survive extremely intenseand prolonged desiccation. There seems to be little phylogeneticlimitation of tolerance in plants but may be more in animals.Physical constraints may restrict tolerance of animals withoutrigid skeletons and to plants shorter than 3 m. Physiologicalconstraints on tolerance in plants may include control by hormoneswith multiple effects that could link tolerance to slow growth.Tolerance tends to be lower in organisms from wetter habitats,and there may be selection against tolerance when water availabilityis high. Our current knowledge of limits to tolerance suggeststhat they pose few obstacles to engineering tolerance in prokaryotesand in isolated cells and tissues, and there has already beenmuch success on this scientific frontier of desiccation tolerance.However, physical and physiological constraints and perhapsother limits may explain the lack of success in extending toleranceto whole, desiccation-sensitive, multicellular animals and plants.Deeper understanding of the limits to desiccation tolerancein living things may be needed to cross this next frontier.     

Desiccation Intolerance in Seeds of Zizania palustris is Not Related to Developmental Age or the Duration of Post-harvest Storage

Freshly harvested seeds of Zizania palustris L. representingfour distinct stages of development were intolerant of desiccation.Rapid drying did not improve survival. The relationship betweenprobit viability and embryo moisture content for freshly harvestedmature seeds was identical to that described in an earlier publicationfor seeds previously stored fully imbibed at low temperaturesto remove dormancy. A small reduction in embryo moisture contentduring the first 3 d of drying at 15% r.h. and 15°C ledto a highly sigificant (P < 0.01) increase in the proportionof fresh seeds capable of germination in response to a rangeof dormancy breaking factors including giberellins. Resultsare discussed in relation to previous work on recalcitrant seedsand to the widely reported developmental changes in orthodoxseeds which are triggered by desiccation. Zizania palustris, wild rice, seeds, drying, desiccation intolerance, development     

Radical Formation and Accumulation In Vivo,In Desiccation Tolerant and Intolerant Mosses

Water loss in a desiccation-sensitive moss resulted in destruction of chlorophyll, loss of carotenoids and increased lipid peroxidation, indicating the presence of damaging forms of activated oxygen. These effects were exaggerated when the plants were desiccated at high light intensities. During water-deprivation there was a build up of a free radical, detected in vivo, with a close correlation between molecular damage and radical accumulation. In contrast, in a desiccation-tolerant moss there was almost no indication of molecular (oxidative) damage. However a stable radical similar in type and concentration to that found in the desiccation-sensitive species accumulated, particularly under high irradiances. The stable radical appears to be one of the end-products of a process initiated by environmental stress, desiccation and high irradiance: its association with molecular damage depending on the degree to which the species is tolerant of desiccation. Identification of the radical in intact tissue from EPR and ENDOR studies, suggests that this is not a short-lived proxy-radical but instead is relatively stable and carbon-centred.     

Heat shock proteins and resistance to desiccation in congeneric land snails

Land snails are subject to daily and seasonal variations in temperature and in water availability and depend on a range of behavioral and physiological adaptations for coping with problems of maintaining water, ionic, and thermal balance. Heat shock proteins (HSPs) are a multigene family of proteins whose expression is induced by a variety of stress agents. We used experimental desiccation to test whether adaptation to different habitats affects HSP expression in two closely related Sphincterochila snail species, a desiccation-resistant, desert species Sphincterochila zonata, and a Mediterranean-type, desiccation-sensitive species Sphincterochila cariosa. We examined the HSP response in the foot, hepatopancreas, and kidney tissues of snails exposed to normothermic desiccation. Our findings show variations in the HSP response in both timing and magnitude between the two species. The levels of endogenous Hsp72 in S. cariosa were higher in all the examined tissues, and the induction of Hsp72, Hsp74, and Hsp90 developed earlier than in S. zonata. In contrary, the induction of sHSPs (Hsp25 and Hsp30) was more pronounced in S. zonata compared to S. cariosa. Our results suggest that land snails use HSPs as part of their survival strategy during desiccation and as important components of the aestivation mechanism in the transition from activity to dormancy. Our study underscores the distinct strategy of HSP expression in response to desiccation, namely the delayed induction of Hsp70 and Hsp90 together with enhanced induction of sHSPs in the desert-dwelling species, and suggests that evolution in harsh environments will result in selection for reduced Hsp70 expression.     

Homoiohydrous (recalcitrant) seeds: Developmental status,desiccation sensitivity and the state of water in axes of Landolphia kirkii Dyer

The desiccation sensitivity in relation to the stage of development was investigated in embryonic axes from the homoiohydrous (recalcitrant) seeds of Landolphia kirkii. Electrolyte leakage, used to assess membrane damage after flash (very rapid) drying, indicated that axes from immature (non-germinable) seeds were the most desiccation-tolerant, followed by those from mature seeds, while axes from seeds germinated for increasing times were progressively more desiccation-sensitive. Differential scanning calorimetry was used to study the relationship between desiccation sensitivity and the properties of water in the tissues. Axes from immature seeds had a lower content of non-freezable water than that of any other developmental stage and a higher enthalpy of melting of freezable water. For mature and immature axes electrolyte leakage increased at the point of loss of freezable water. At other developmental stages the water content at which electrolyte leakage increased markedly correlated with the other properties of the water, such as the change in the shape of the melting endotherm and the onset temperature. Ultrastructural studies of axes at the various developmental stages showed changes in the degree and pattern of vacuolation, the presence and quantities of lipid and starch, and the degree of endomembrane development. The results are discussed in relation to current hypotheses on the basis of desiccation tolerance.Abbreviation DSC differential scanning calorimetry